Heretofore, an alkali chloride electrolysis method using an ion exchange membrane method has been known, which electrolyzes an alkali chloride aqueous solution using a fluorine-containing cation exchange membrane as a separation membrane, to produce an alkali hydroxide and chlorine. Further, as such a fluorine-containing cation exchange membrane to be used for electrolysis of an alkali chloride, heretofore, a cation exchange membrane comprising at least two layers that are a layer made of a fluoropolymer having sulfonic groups disposed on an anode side and a layer made of a fluoropolymer having carboxylic groups disposed on a cathode side, has been suitably employed.
In an ion exchange membrane to be used in such an electrolytic cell for electrolysis of alkali chloride, there has been a problem that deterioration of strength of the peripheral portion of the membrane in the vicinity of a clamped portion, particularly in the upper portion of the electrolytic cell, occurs during long term use. The deterioration of strength in the vicinity of the clamping portion is due to a mechanical cause. However, the deterioration of the strength of the membrane in the upper portion of the electrolytic cell is considered to be caused because alkali hydroxide is diffused from the cathode side into the ion exchange membrane in the upper portion of the electric cell where a chlorine gas tends to be accumulated, and the alkali hydroxide reacts with the chlorine gas diffused from the anode side, to deposit alkali chloride, which causes deterioration of the strength of the membrane.
For the purpose of preventing the deterioration of the strength of an ion exchange membrane, there has been proposed a method of covering an upper portion of the ion exchange membrane by an non-permeable film having low ion exchange capacity (refer to Patent Document 1), a method of carrying out a hydrophobizing treatment such as application of a grease to the peripheral portion of the ion exchange membrane (refer to Patent Document 2), and a method of increasing the total thickness of only the peripheral portion of the ion exchange membrane (refer to Patent Document 3).
However, in these methods, there has been such problems that it is necessary to carry out a troublesome shaping process or a treatment operation to an ion exchange membrane, and when the film thickness over the entire membrane is uneven, wrinkles or breakage occurs at a time of rolling in the production.
Here, as a method of adjusting the components of the cation exchange membrane itself to prevent the deterioration of the strength, a method of increasing the thickness of the layer made of a fluoropolymer having carboxylic groups among two layers of the cation exchange membrane, can prevent the deterioration of the strength of the peripheral portion of the cation exchange membrane particularly in the upper portion of the electrolytic cell. However, in this case, the membrane resistance of the entire cation exchange membrane increases, such being not preferred. Further, if the thickness of the layer made of a fluoropolymer having carboxylic groups is reduced for the purpose of reducing the membrane resistance, besides the above deterioration of the strength of the peripheral portion of the membrane particularly in the upper portion, there has been a problem that an alkali chloride diffuses from the periphery of the cation exchange membrane toward the cathode side to deteriorate the quality of product alkali hydroxide.
Under the circumstances, there has been desired a cation exchange membrane for electrolysis of an alkali chloride, which can maintain a good production efficiency when it is employed for the electrolysis, and which can prevent deterioration of the strength of the peripheral portion, particularly along the upper end portion, of the membrane, and which can be produced easily with low cost.